Throughout the Indian Ocean, niches formerly occupied byAcroporaare being progressively replaced by corals with different life history strategies, including previously subordinate taxa, as well as by non-carbonate accreting producers such as seaweeds and zooanthids (Sheppard and Obura 2005; Harris et al.2010). Many of the observations of dramatic reductions inAcroporaalso apply toMontipora. Coral assemblages have been observed to shift towards conditions of dominance by faviids andPoritesat numerous sites in the region (Edwards et al. 2000; Loch et al. 2002; Spencer et al. 2000).
Surveys carried out in the aftermath of the 1998 coral bleaching event in the Maldives, where the extent of coral mortality was amongst the highest in the world showed the encrusting agarididPavona variansto be the main constituent of the recruit
community, and recorded a shift in dominant species in the overall coral community from acroporids and pocilloporids to agaricids (Schuhmacher et al. 2005). The same study at Fadiffolu atoll from 2000 to 2002 showed the relative composition of Agariciidae to increase from 49% to 74%, whileAcroporaand Pocilloporidae
decreased from 22% to 3% and 17% to 1% respectively (Schuhmacher et al. 2005). Edwards et al. (2000) recorded a post-bleaching shift in species composition in the Maldives “from a 95% pocilloporid / acroporid dominated coral community to a 92% agariciid / poritiid / faviid one” (Edwards et al. 2000). Similar post-bleachingPavona
dominance was recorded elsewhere in the Maldives in the aftermath of the mortality episode (Loch et al. 2002; McClanahan 2000). Other transects carried out in the Maldives 21 months after the 1998 mortality episode identifiedGoniastreaspp.,
Porites,Favites flexuosaandPodabacia crustaceaas the dominant recolonising coral species (Loch et al. 2002).
Studies in the Maldives in 1999 and 2000 showed thatAcropora palifera,Stylophora,
Seriatopora, branchingPoritesandMillepora(P. nodifera,P. compressa,M. teneraand
M. intricata) may have been locally extirpated, while the surviving dominant corals were massive and columnar Poritidae, Faviidae, and Acroporidae (Astreopora myriophthalmaandPorites lutea,Porites cylindricaandP. rus) (Loch et al. 2002;
McClanahan 2000). Surveys carried out 21 months after the 1998 mortality episode identifiedPavona,Leptoseris,Goniastreaspp.,Porites,Favites flexuosaand the brooding coralPodabacia crustaceaas the dominant survivors and recolonising coral species. These studies also showed that agariciids, faviids and poritiids (notablyPavona,
Leptoseris Coscinarea,PoritesandSynarea), dominated the recruit community during the early stages of recolonisation (1999), withAcropora,MontiporaandPocillopora
recruits being comparatively scarce (Loch et al. 2002; McClanahan 2000; Schuhmacher et al. 2005). Roughly one third of new settlers recorded in one study were agariicids (Loch et al. 2002). Observations that the dominant corals in 1999 differed from the pre-mortality coral community led to the hypothesis that Maldives reefs might be following a recovery trajectory favouring coral genera that did not dominate the pre- 1998 reef community (McClanahan 2000).
In Chagos the near total elimination ofA. palifera, formerly the dominant wave-
resistant architectural reef crest species throughout the archipelago, resulted in a drop of shallow reef height of approximately 1.5 m (Sheppard 2006). Like Chagos, many Seychelles reef flats were covered, pre-1998, with 0.5 m stagshornAcroporathat reached low tide level. These thickets are now dead in most areas (Sheppard 2006; Sheppard et al. 2005).
In the Granitic Seychelles coral bleaching in early 1998 led to mortality of 85-95% of coral cover, with the generaAcroporaandPocillopora, principal architectural
components of the reef structures, suffering the highest levels of mortality. Following subsequent bleaching events in 2002 and 2003, slow growing corals such as massive
PoritesandGonioporawere observed to become increasingly dominant genera, suffering only temporarily arrested growth from bleaching events that brought about significant losses in branching and encrusting taxa (Engelhardt 2004; Engelhardt et al. 2002).
Surveys in the southern Seychelles during the 1997-1998 Indian Ocean warming event recorded high levels of bleaching and mortality in branching coral species, in contrast to low levels in encrusting species. Large stands ofAcroporaandPocilloporaspp. displayed levels of up to 100% bleaching, compared with only partial bleaching in
Alphonse islands of the southern Seychelles in 2005 showed live coral cover to be dominated byPoritesandPocillopora(Hagan et al. 2008). Other studies of coral bleaching in the southern Seychelles in 1998 showed far higher bleaching incidence in
Acropora,Pocillopora,GalaxeaandSeriatoporathan in massive corals such asPorites
andPavona(Spencer et al. 2000).
Corals in Cosmoledo, in the Aldabra group of the outer Seychelles islands, suffered almost total mortality to approximately 10m depth, with around 50% mortality in deeper zones, varying depending on coral taxa. Unlike Acroporid and fungiids, which showed near-ubiquitous mortality, Poritiids and mussids showed appreciable survival of adult corals (Sheppard and Obura 2005). Coral recruitment in Cosmoledo was observed to have recovered rapidly four years after the mortality episode. However, the recruit community was dominated by faviids andPorites, which were the most successful surviving taxa. Conversely, the recruit community showed a scarcity of certain previously dominant genera, notablyAcropora, which had experienced the most serious mortality at all depths surveyed during the study. The results of these surveys resulted in predictions of long-term shifts in identity of dominant coral species of corals in these atolls (Sheppard and Obura 2005).
Surveys of coral reefs within the marine reserves of northern Madagascar, from 2005- 2006, show meanAcroporacover to be less than 1.7% at all marine parks with the exception of Cap Masoala (4.5%). The mainAcroporaspecies recorded were submassive and encrusting forms, despite evidence at several sites of large, intact
Acroporaskeletons assumed to be relics of an earlier mortality episode, probably occurring in 1998. The vast majority of non-Acroporacorals comprised encrusting or massive forms (Harding and Randriamanantsoa 2008).
In the Arabian Gulf immediately following the 1998 event, coral cover in shallow water (<3 m depth) was less than 1% due to near total mortality of vast areas of shallow
Acroporacorals. Her too, faviid corals dominated the juvenile assemblages, as opposed to previously-dominantPoritesandAcropora. These observations led to conclusions that the region’s reefs had experienced a shift in their coral communities towards an alternative stable state dominated by faviids. The authors hypothesised that, given the
extreme temperature regimes experienced in the Arabian gulf, the observations
recorded may be indicative of likely future changes in other areas of the broader Indian Ocean (Sheppard and Loughland 2002).
Acropora,formerly the most abundant and diverse genus in the Arabian Gulf, has become a rare genus in some areas, with near-total removal of stagshorn and tabular forms in most shallow areas, many of which have been replaced by faviids (Sheppard 2006). Surveys of reefs in the Arabian Gulf following the 1998 mortality event
recorded fewer than twenty coral species altogether (Sheppard and Loughland 2002). Studies of the temperature tolerances of hard corals from the Arabian Gulf suggest that branchingAcropora,Stylophoraand a branchingPorites(P. nodifera) are those most vulnerable to temperature changes (Sheppard et al. 2000).
Similarly, on Kenyan reefs in 2007,Pocilloporashowed the most bleaching-related mortality of corals at all sites studied. Poritescolonies also experienced widespread bleaching, but unlikePocilloporathis genus showed negligible levels of subsequent mortality (Grimsditch et al. 2008).
McClanahanet al.(2004) found significant genus-based responses to bleaching and mortality between reef sites in Kenya and Australia;Millepora,Stylophoraand
Pocilloporawere consistently susceptible, whereasCyphastrea, Goniopora,Galaxeaand
Pavonawere consistently resistant (McClanahan et al. 2004). A western Indian Ocean- wide study of coral community structure, bleaching and susceptibility carried out across 8 countries in 2005, predicted that low-diversity genera with narrow environmental ranges would be most vulnerable to future extinction. Such genera includeGyrosmilia interrupta,Plesiastrea versipora,Plerogyra sinuosa, andPhysogyra lichtensteini(McClanahan et al. 2007). Observations from the Chagos islands in 2006 suggest that the formerly common monospecific genusDiploastreamay have suffered local extinction whilst others, notably Montipora, had become extremely rare relative to previously recorded levels. The non-scleractian hydrozoanMilleporaand blue coral
MassivePoritesandPavonaare considered to be resistant to bleaching relative to most other genera (Mumby et al. 2001; McClanahan et al. 2004). Branching and columnar
Poritesalso display high tolerance to bleaching (Grimsditch et al. 2008), and to be successful recruiters after bleaching events (Loch et al. 2002; McClanahan 2000).
Pavonawas the only genus noted to show resistance to bleaching-related mortality in Chagos in 1999 (Sheppard 1999).OxyporaandPachyserishave also been observed to show bleaching tolerance (CRC Sheppard pers. comm. in McClanahan et al. 2007). However, althoughPoriteswas a dominant survivor at many sites throughout the Indo- Pacific (including the Arabian Gulf) it was heavily impacted too, with branching
poritiids exhibiting high degrees of mortality at some sites, and massive colonies showing intermediate bleaching, albeit with higher recovery frequency than branching corals (Kayanne et al. 1999). Poriteswas also the dominant dead species at some sites in the Arabian Gulf (Sheppard 2006).